1. Technical Field
The present invention generally relates to a multi-layer sleeve for use on a cylinder of a rotary press, wherein the sleeve is expandable, i.e. its inside diameter can be increased. The invention particularly relates to a sleeve designed for flexographic printing, specifically for so-called variable-sleeve offset printing. The invention also relates to a corresponding manufacturing method.
2. Prior Art
Such sleeves are generally used to change the printing plate rapidly and inexpensively. They eliminate the need for changing the rotating, driven printing press cylinder itself. In the prior art, the term “sleeve” encompasses both adapter sleeves, which are mounted on the cylinder to carry a further, thinner-walled sleeve, and also sleeves with an actual plate or plate holder, to which a typically flexible printing plate is applied. Also known are sleeves where the plates are engraved directly into the outer surface, e.g. by laser, or applied in some other way. Furthermore sleeves are known whose surfaces serve as a cushioning, intermediate layer to accommodate comparatively rigid printing plates with the actual printing plate. The present invention is fundamentally applicable both to the first two sleeve types and to the two last mentioned types.
The sleeves are usually slid onto the carrier body, i.e. the printing press cylinder, by generating an air cushion between the sleeve and the cylinder. To this end, the printing press cylinder typically has air outlet openings on its surface that are only pressurised to mount and dismount the sleeve. The sleeves are designed to be expandable to this purpose, i.e. with an expandable inside diameter. Multi-layer sleeves have found widespread use in this context, in order to guarantee stability during operation, and particularly slip-free fixing on the cylinder by means of a non-rotating, frictional connection, despite the expandable inside diameter.
A multi-layer sleeve designed specifically for flexographic printing is known from European Patent Application EP 1 361 073, for example. This sleeve encompasses, in a typical design, an inner layer with an inner jacket of comparatively high strength and/or surface hardness that permits frequent mounting and dismounting. The inner jacket is nevertheless elastically expandable to a minimum degree for mounting and dismounting. Expandability is additionally enabled, in wholly conventional manner, by a compressible layer being specially provided, radially outside the inner jacket. The compressible layer is customarily a relatively thin-walled layer of foamed polymer and permits reversible expansion of the inner jacket. The sleeve according to EP 1 361 073 has an intermediate layer on the outside of the compressible layer. The customary intermediate layer determines the wall thickness or the overall diameter of the sleeve and is designed with a corresponding thickness. Particularly in the case of relatively large wall thicknesses, the intermediate layer should be made of the lightest possible material, such as foamed polyurethane. According to the conventional design, as also proposed in EP 1 361 073, the intermediate layer is, in contrast to the compressible layer, designed to be as rigid as possible, i.e. radially incompressible. The intermediate layer is then followed by an outer layer. The latter, in turn, typically has an outer jacket that has the hardest possible surface and is usually not expandable.
With the aim of increasing the service life and surface quality (TIR), U.S. Patent Application US 2002/0046668 proposes a multi-layer, expandable sleeve that comes entirely without a conventional compressible layer. The compressible layer is eliminated in that, as a result of an appropriate design, the intermediate layer itself permits a certain degree of elastic expansion of the inner jacket, e.g. in the range of approx. 4 to approx. 12 hundredths of a millimeter. For an intermediate layer of this kind, US 2002/0046668 recommends a special polyurethane with a Shore D hardness between approx. 45 and 50.
Patent Specification EP 0 683 040 likewise discloses a sleeve without a compressible layer on the inner jacket. The sleeve according to EP 0 683 040 also displays a multi-layer structure with an inner and outer jacket and an intermediate layer. Apart from the absence of a separate compression layer, this sleeve has—with a view to saving weight and in contrast to US 2002/0046668—an intermediate layer of special, lightweight honeycomb design with radially oriented honeycomb webs (see FIG. 3). This intermediate layer comprises layers of matting/resin composite, between which at least one hexagonal honeycomb structure is formed, consisting of cured polymer resin. The hexagonal structure is produced integrally from polymer resin. To this end, a strip of special matting that is specifically suitable for this purpose with impressed channels and which does not itself absorb resin during impregnation, is wound around the matting layers in spiral fashion. The matting layers and the channels in the special strip are subsequently coated with resin, such that the matting layers are impregnated and the hexagonal channel structure is filled with resin. After curing, this results in a special sandwich structure with honeycomb webs consisting exclusively of polymer resin. These webs form bridges between the matting/resin face layers. According to EP 0 683 040, pillar-like resin bridges are additionally provided within the non-impregnated hexagonal cells made of matting. They are produced by radial holes in the special matting, distributed around the circumference, which are likewise filled with resin. According to EP 0 683 040, the special design of the intermediate layer itself, with hexagonal webs and supports made of resin between the matting/resin composite layers, is essential so that the inner jacket can expand radially away from the longitudinal axis. According to EP 0 683 040, several hundredths of a millimeter, i.e. at least 20 μm, expansion in the radial direction are said to be guaranteed.
The disadvantages of a sleeve according to EP 0 683 040 are at least the relatively complex process and the nevertheless comparatively high density i.e. volumetric weight of the special intermediate layer.